Pectin is a complex mixture of colloidal polysaccharides found in the primary cell walls of both monocotyledons (monocots) and dicotyledons (dicots) in the plant kingdom. The main polysaccharide structures are homogalacturonan (HGA, commercial food use pectin), rhamnogalacturonan-I (RG-1), and substituted galacturonans. HGA is a linear chain of 1,4-linked α-D-galactopyranosyluronic acid (GalpA) residues in which some of the carboxyl groups are methyl esterified. Rhamnogalacturonan has been called the “hairy region” of pectin. Rhamnogalacturonan-I (RG-1) is a family of pectic polysaccharides that contain a backbone of the repeating disaccharide [→4)-α-D-GalpA-(1→2)-α-I-Rhap-(1→]. 20-80% of the rhamnosyl (Rhap) residues are, depending on the plant source and method of isolation, substituted at C-4 with neutral and acidic oligosaccharide side chains. The predominant side chains contain linear and branched α-I-arabinofuranosyl (Araf), and/or β-D-galactopyranosyl (Galp) residues although their relative proportions and chain lengths may differ depending on the plant source. Some of the side chains may be terminated with α-L-Fucp, β-D-GlcpA, and 4-O-methyl β-D-GlcpA residues. The backbone GalpA is not typically substituted with oligosaccharides. The backbone GalpA residues are O-acetylated in rhamnogalacturonans from many plants. Rhamnogalacturonan-II (RG-II) is not structurally related to RG-1 since its backbone is composed of 1,4-linked-D-GalpA. RG-II has nonasaccharide and octasaccharide side chains attached to C-2 of some of the backbone GalA residues and two structurally different disaccharides are attached to C-3 of the backbone.
Rhamnogalacturonan-I (RG-1) has been previously isolated from many plants and vegetables including potato. Isolated Rhamnogalacturonan-I is typically a polysaccharide with a molecular weight in the 100,000 Da-1000,000 Da size range. When pure, RG-1 is typically soluble in water with low intrinsic viscosity. RG-1 has been discussed to have mitogenic, immunostimulation and antitumor activity.
As the structure, size and composition of RG-1 determines its bioactivity, small changes in structure may confer large changes in activity. Improved selective extraction and purification technologies are required if products are to be available on a large enough scale for health applications. Whilst previous examples of pectin and RG-1 extraction methods have been disclosed for sources such as bell pepper, carrot and apple (US2014/0056946, WO2012/148277), and for sweet potato (CN101627825), they described an alcohol or sodium hydrogen phosphate-based precipitation and purification process. An example of the process of fragmentation of a pectin molecule using free radical depolymerisation, has been disclosed (WO2013/101314), generating a specific mixed HGA-RG-1 product.
Examples of immunomodulatory, mitogenic and antitumour activity of RG-1 containing polysaccharides and oligosaccharides have been disclosed for molecules derived from bell pepper, carrot and apple (US2014/0056946, WO2012/148277) from ginseng (US2012/0278945), from the tea plant Camellia sinensis (US2013/0064858; WO2011/069781) and from citrus and other plant sources (US2011294755; WO2013/101314).